1. Field
The present disclosure relates to wireless communication systems, and more particularly to a method and apparatus for channel estimation in a wireless communication device.
2. Background
A wireless communication system can be described by the equationyi=√{square root over (Es)}hixi+ni,
where xi is the transmitted symbol, hi, is the fade, or channel estimate, ni, the noise and yi is the received signal at time i. Es is the signal power and NO is the noise power (E|ni|2).
The fades hi are not known to the receiver and need to be estimated. This is usually done by having transmitting known “pilot symbols” pi from which the receiver can estimate hi.zj=√{square root over (Ep)}hjpj+nj 
The process of estimating hj from the corrupted pilot symbols zj Is known as channel estimation.
In general, the power Ep at which the pilot symbols are transmitted differes form the power Es as which the data symbols are transmitted. The ratio Es/Ep is known as the Traffic-to-Pilot ratio or T/P ratio.
Often the T/P ratio is not known to the receiver. Furthermore, it can vary from time to time and from user to user in a multi-user system. This could occur for instance when the transmitter is employing power control. However, the receiver needs to know Es accurately to be able to correctly demodulate the received signal and recover the transmitted data accurately.
One option is to transmit the value of the T/P ratio over an overhead control channel. However, this is expensive in terms of overhead channel bandwidth. It would be beneficial if the receiver could estimate the traffic energy from the received data yi. Estimating the T/P ratio is called T/P estimation or traffic energy estimation.
An example of a communication system that can benefit from T/P estimation is an orthogonal frequency division multiple access (OFDMA) system. An OFDMA system utilizes orthogonal frequency division multiplexing (OFDM), which effectively partitions the overall system bandwidth into a number of (N) orthogonal frequency subbands. These subbands are also referred to as tones, sub-carriers, bins, frequency channels, and so on. Each subband is associated with a respective sub-carrier that may be modulated with data. An OFDMA system may use any combination of time, frequency, and/or code division multiplexing.
For an OFDMA system, multiple “traffic” channels may be defined whereby (1) each subband is used for only one traffic channel in any given time interval and (2) each traffic channel may be assigned zero, one, or multiple subbands in each time interval. The traffic channels may include “data” channels used to send traffic/packet data and “control” channels used to send overhead/control data. The traffic channels may also be referred to as physical channels, transport channels, or some other terminology.
The traffic channels for each sector may be defined to be orthogonal to one another in time and frequency so that no two traffic channels use the same subband in any given time interval. This orthogonality avoids intra-sector interference among multiple transmissions sent simultaneously on multiple traffic channels in the same sector. Some loss of orthogonality may result from various effects such as, for example, inter-carrier interference (ICI) and inter-symbol interference (ISI).
In a wireless communication system such as the OFDMA system, it is often necessary to estimate the response of a wireless channel from a transmitter to a receiver. The channel estimate may be used for various purposes such as data detection, time synchronization, frequency correction, spatial processing, rate selection, and so on. Channel estimation is typically performed by transmitting a pilot signal containing pilot symbols that are known a priori by both the transmitter and receiver.
The pilot signal is typically impaired by both noise and interference. These impairments degrade the quality of the channel estimate obtained by the receiver based on the received pilot signal. The noise can come from various sources such as the wireless channel, receiver properties, and so on. Noise impairment can normally be addressed by transmitting the pilot signal in a proper manner and/or for a sufficient period of time such that the receiver can obtain the desired quality for the channel estimate. The interference can result from multiple transmitters transmitting their pilot signals simultaneously. These transmitters may be for different base stations in the system, different antennas of the same base station, and so on. The pilot signal from each transmitter may act as interference to the pilot signals from other transmitters. This pilot interference degrades the quality of the channel estimate.
It is often desired to estimate the channel and the level of interference. On the forward link (FL), common pilot symbols are known to have been used. In the OFDMA system, such common pilot symbols are typically scattered over the entire bandwidth shared by all the users. In a traditional single-antenna transmission, such common pilot symbols may be exploited by all the users for the purpose of FL channel estimation. The bandwidth and channel coherence time values that are typical in cellular applications render common pilot tones particularly useful.
The relative bandwidth efficiency of the common pilot versus dedicated pilot may be made by a comparison between the total number of degrees of freedom in a broadband channel corresponding to the total shared bandwidth, estimated with the common pilot, and the number of degrees of freedom in a narrow-band sub-channel allocated per user times the number of such narrow-band sub-channels. For bandwidth and channel coherence time values that are typical in cellular applications, this balances in favor of the common pilot. Nevertheless, the dedicated pilot approach has a number of attractive features.